Tube current stabilizer



Nov. 4, 1952 M. R. cos

TUBE CURRENT STABILIZER Filed Jan. 8, 1949 INVENTOR Patented Nov. 4, 1952 TUBE CURRENT STABILIZER Myron R. Coe, Richmond Hill, N. Y., assignor to Philips Laboratories, Inc., Irvington-on-Hudson, N. Y.

Application January 8, 1949, Serial No. 69,937

4 Claims. (01. 250-97) My invention relates to a device for stabilizing the current flowing through an X-ray tube.

It is an object of my invention to provide a current stabilizer for an X-ray tube which is independent of supply voltages.

It is a further object of my invention to provide a current stabilizer which is inherently sensitive to small changes in tube current in an X- ray tube.

It is a still further object of my invention to provide a current stabilizer which is relatively simple to construct.

These and further objects of my invention will appear as the specification progresses.

According to my invention, the tube current of an X-ray tube is stabilized by controlling an impedance in the filament supply circuit for the tube. This is accomplished by deriving a potential which is proportional to the tube current and applying this potential to a control electrode of a multi-element thermionic discharge tube, the output of which reflects an impedance proportional to the potential applied to the control electrode. This output impedance is thereupon reflected into the filament supply circuit of the X-ray tube so that a change of anode current'is reflected as a change in impedance in the filament supply circuit.

In order that the invention can be more clearly understood and carried into effect, it will now be described in connection with the appended drawing in which the single figure shows a current stabilizer for an X-ray tube in accordance with the invention.

An X-ray tube I provided with an anode 2 and a filamentary cathode 3 is supplied with high tension excitation through a high tension transformer 4 having a split secondary winding. Be-

tween the two sections of the secondary winding a resistor 5 develops a potential proportional to the tube current which potential is applied to the control electrode of a multi-element thermionic discharge tube 6 through a resistor 1 and a glow discharge lamp 8. While I prefer to use a beam tetrode of the SL6 type, any multi-element tube having a substantially constant plate impedance may be employed such as the conventional pentode tube.

The output of tube 6 is coupled through a plate supply transformer 9 to a potential source It such as an auto transformer connected to the power supply and provided with connections for supply voltage to the high tension transformer 4 and the filament supply transformer IS. The secondary winding of transformer 9 is connected I in series with the primary winding of transformer I 6 so that a change in the output impedance of tube 6 is reflected as a change in impedance in the filament supply circuit.

The screen electrode of tube 6 is supplied through a separate transformer winding 1 I from a power source through a screen supply resistor l2 and a voltage regulator type tube 13 which maintains the screen voltage independent of supply voltage fluctuations.

To understand the operation of the stabilizer, it is assumed that the X-ray tube is connected in the circuit of the high tension supply and is energized. An anode current flowing through tube l causes a potential drop across resistor 5 which biases 6 at some point on its characteristic curve. Tube 6, which is energized by an A.-C. potential through tranformer 9 reflects a given impedance into the output circuit of transformer 9 and thence into the filament supply circuit of the X-ray tube. If the anode current changes due to changes in supply voltage or 10- oalised heating of the filament, a corresponding potential change occurs across resistor 5 thus altering the bias on tube 6. As a result of a change in tube bias, tube 6 changes its output impedance which is reflected into the filament supply circuit thus controlling the heating current for the filament. Thus, an increase in tube current is reflected as an increased impedance of the tube which in turn is reflected as an increase in impedance in the filament supply circuit.

In order to increase the sensitivity of the stabilizer, a filter consisting of a resistor l4 and a capacitor l5 shunt the control electrode to the cathode of the tube between the glow discharge tube 8 and the control electrode. Resistor I4 and glow discharge tube 8 form a potentiometer in which the glow discharge tube represents a non-linear resistance. At the breakdown voltage of the glow-discharge tube it conducts and at higher voltages, the tube has a constant potential drop. Hence, voltage difference between the voltage available across resistor 5 and the tube breakdown voltage appears across resistor M which now serves as a voltage amplifier.

Further, since tube l is of the self-rectifying type, current flows only during alternate half cycles through the tube causing voltage pulses to appear across resistor 5. With capacitor l5 charged during the voltage pulse, the capacitor slowly discharges through resistor M during that portion of the cycle when tube 1 is non-conmionic discharge device including a cathode, an.

anode and a control electrode, a cathode-anode supply circuit for said X-ray tube, means coupled to the cathode-anode circuit otthe X-ray tube for deriving a control potential proportional;

to the current flowing therein, means to apply said control potential t the. control electrode.

of the thermionic discharge device to vary the internal resistance thereof accordingly, an,.al.-, ternating current supply circuit for the cathode of said X-ray tube, an impedance. interposed in the said supply circuit, and means couplingthe cathodeand r anode of the. thermionic; discharge device in shunt relation with-.saidiimpedance to derive an alternating-current. voltagefor energizing the said thermionic discharge, device whereby the internal resistance-of the saidthermionic discharge device. is reflected into. said supply circuit.

2. A current stabilizedfor an Xeraytube having a cathode and an anode comprising athermionic discharge device includinglacathode, an anode and a control electrode, a cathode-anode supply circuit for said X-raytube, animpedance'in the cathode-anode circuit ofthe Xeray 0 tube. for deriving a control-potential proportionaltoithe current flowing, therein, means to. apply the said .control potential to the: control electrodeof thethermionic dischargedevice to vary the internal resistance accordingly, an alternating current supply circuit for the-cathode of said X-ray tube, a transformer having intercoupled windings one of which isinterposed in .the supply circuit, and: means coupling, the-cathode and anode-oi theldischarge, device inzshunt relation withanother of the windingsaoisaid transformer to=derive an alternating-current voltage for en,- ergizing' the said thermionic discharge device whereby the internal resistanceofthe said thermionic discharge device is reflected into said supply circuit.

3. A current stabilizer for an X-raytube having a cathode and an anode comprising a thermionic discharge device includinga cathode, an

anode and a control electrode, an alternatingcurrent cathode-anode supply circuit for said X- ray tube, an impedance in the cathode-anode circuit of the X-ray tube for deriving unidirectional voltage pulses proportional to the current flowing therein, means to derive a control potential from said voltage pulses, means to apply the control potential to' the controlrelectrode of the thermionic discharge device, an alternating current supply circuit for the cathode of the Xray tube, a transformer having intercoupled windings oneof which is interposed in the supply circuit; and" means coupling the cathode and anode: ofthe. discharge device in shunt relation withanother of Ltheewindings of said transformer to derive an alternating-current voltage for energizing the said. thermionic discharge device whereby the internal resistance of the said thermionic; discharge device is reflected into said supply circuit.

4; A current. stabilizer; for. an. X-raytube having. a cathode and. an .anode comprising: a theremionic. discharge; device including, a. cathode,-,-an anode and atcontrolelectrode, an alternatingcurrent cathode-anode supply-circuitfor. saidrXe ray tube, an impedance in-the. cathode-anode circuit of the X-ray tube for deriving unidirectional voltage pulses proportional tothe current flowing therein, means to derive a control potential from said voltage pulses: comprising a pulse'filter networkincludinga threshold limiter, means to apply the control potential to the control electrode of. the thermionic discharge device, analternating current supply circuit for the cathode of said X-ray tube, a transformer having intercoupled windings oneof which isinterposed in the. supply circuit, and means coupling the cathodeand. anodelof the-discharge device in shunt relationwith another of said.

windings to, derive analternating-current voltage for energizing, the discharge-device wherebythe internal resistance. ofthe discharge, device.- is reflected into said supply circuit MYRON RHCQE.w

REFERENCES f CITED The following references are of record in thefile of this patent:

UNITED STATES PATENTS Number Name Date 2,319,378. Wisglass May- 18,1943 2,512,193 Zavales iJune.20, 1950 

